1. Field of the Invention
This invention relates to pneumatic brakes for rail vehicles and, more particularly, to an emergency brake release timer for delaying the application of emergency braking pressures.
2. Description of Related Art
It is well known in the brake control art that modern train brake control systems typically use a central controller unit to control the brakes of the rail vehicles that comprise the train. A train operator located in the lead cab manipulates the brake handles or like devices of the train to apply and release the brakes of the trucks or bogies on each rail vehicle as desired. The inputs from the brake handles are typically processed by a cab control unit and passed to the central controller unit. In response to these and other inputs, the central controller unit issues a brake command signal along a train line to each of the rail vehicles in the form of either a pneumatic signal or an electrical signal or even both. Brake equipment on each of the rail vehicles applies or releases the brakes according to the dictates of the particular brake command signal received. The central controller unit sends the brake command signal to each of the rail vehicles along either a pneumatic train line or an electrical train line.
Depending on the type of train being considered, the brake equipment on each rail vehicle may include either exclusively pneumatic equipment or a combination of electrical and pneumatic (i.e., electro-pneumatic) equipment. In those trains featuring electro- pneumatic brake equipment on the rail vehicles, each rail vehicle typically includes a local control system whose construction and operation are generally well known in the brake control art. The local control system on each rail vehicle receives the brake command signal and various other signals in response to which it directly controls the electro-pneumatic equipment according to principles well known in the brake control art. Specifically, the local control system generates the electrical signals which open or close the various valves which supply pressure to or vent pressure from the brake cylinders. The brakes on each rail vehicle apply and release accordingly.
The prior art brake control systems discussed in the foregoing paragraphs typically provide service braking, emergency braking, dynamic brake blending, and wheel slip control functions. The local control system includes a microprocessor-based device which performs service brake control, limited emergency brake control, dynamic brake blending, and wheel slip control. Associated with the brakes on each bogie is a Brake Cylinder Control Unit (BCCU) which manipulates brake cylinder pressure based on pneumatic and/or electric inputs from the local control unit. It provides service and emergency brake cylinder pressure control as well as dynamic brake blending and wheel slip brake cylinder pressure modulation. The BCCU has integrated magnet valves and transducers which are monitored and controlled by the local control unit. The primary functional element of the BCCU is a Variable Load Relay Valve (VLRV) that provides a controlled pneumatic pressure to the brake cylinders. The output of the VLRV is a function of the pressure at its pilot pressure control port and an input indicative of vehicle load. The pilot pressure commands brake cylinder pressures during service braking. The pressures are related, for example, to speed and vehicle weight. In an emergency, the pilot pressure port of the VLRV is controlled for emergency stopping which may include modulation of brake cylinder pressure to minimize wheel slipping. Typically, the source of pilot pressure commands for service braking and emergency braking are different and are selected by an Emergency Magnet Valve (EMV) controlled by the local control system.
Typically during full emergency braking, modulation of the braking pressure by the VLRV or wheel slip control is inhibited. Inhibiting brake pressure modulation to wheel slip control often results in wheel slides and wheel lockups with extended stopping distances. With increasing demand for wheel protection in all brake modes along with increased emphasis on system safety, the pneumatic brake release timer according to this invention becomes necessary and easy to configure and analyze. The pneumatic brake release timer according to this invention will:
1. Provide a means to reduce the emergency brake level for a fixed limited period of time which begins at the start of the emergency condition and is not resetable.
2. Provide a non-electronic apparatus to return emergency brake pressure to the required level.
3. Allow for limited protection of the wheels in an emergency brake condition.
4. Allow limited duration software-controlled wheel slip control of emergency braking modes.
5. Provide a known reliability analysis of the emergency brake modes.